(1) Field of the Invention
This invention relates to the control of radiant thermal energy and more specifically to highly Thermal Infrared (IRT) reflective pigments for use in decorative coatings for inhabited buildings or other areas where the control of IRT energy and visual decoration are required.
(2) Description of the Art
Control of the spread of thermal energy in domestic buildings through passive techniques reduces energy consumption by reducing reliance on heating in cool environments and cooling air-conditioning in warm environments.
Convection as ‘draughts’ and conduction are widely recognised heat transport mechanisms and many materials and methods have been developed to reduce their effect on energy consumption in structures. Thermal energy may also be transported through radiative processes. Humans readily perceive conduction and convection processes but are less sensitive to radiative heat transfer from surfaces at near ambient temperatures. It is possible to control radiative thermal energy transport processes to reduce or promote heat transfer into a room using appropriate surface coating materials. For example metal reflectors, usually aluminium, are used in roof and wall cavity insulation materials where a layer of the metal is bonded to the insulation surface to reduce IRT energy radiating from the surface. However, the use of such highly IRT reflective materials is currently restricted to areas where decorative appearance is not a primary concern, for example in loft spaces and in cavity wall voids.
Conventional, unmodified, decorative paints typically comprise a mixture of colour pigments in a solvented, optically clear film-forming material known as the binder. The binder is conventionally an organic polymer in decorative coatings for inhabited environments. Paints may also contain a wide range of additional materials such as flow improvers, wetting promoters etc., in small quantities. In addition to binding the colour pigments to the substrate, the binder also provides other desirable properties such as the gloss, abrasion resistance and corrosion or biological attack resistance.
A number of paint types based on variants of conventional decorative paint fluids with increased IRT reflectivity exist in the literature.
Type 1 IRT reflective coatings are based on scattering granular pigments chosen and graded to maximise reflectivity in the IRT waveband. U.S. Pat. No. 5,811,180 (Paul Berdahl, “Pigments which reflect radiation from fire”) describes a paint of this form.
However, these formulations have relatively low reflectivity in the IRT waveband due to absorption in the pigment particles, multiple reflections leading to long path lengths through the organic polymer binder and poor scattering due to small differences in refractive index of the binders and conventional granular pigments at IRT wavelengths. Reflectivities in the IRT waveband of greater than 0.3 are difficult to achieve in coloured coatings based on scattering granular pigments.
Type 2 highly IRT reflective paints have been developed that use metal flake pigments to provide the IRT reflector component. EP0065207 (Herberts & Co GMBH (DE), “Use of pigmented coating compounds with reduced emision capability in the spectral range of the heat radiation for camouflage purposes”) describes a coating of this form. Typically aluminium flake in the size range 10 to 50 microns diameter is used as the IRT reflective pigment. Such metal-flake and binder only paints can be formulated with high reflectivities in the range 0.7 to 0.75. Using metal flakes with a surface treatment that instills a tendency to congregate and orientate at the binder outer surface (or leaf), and a binder chosen for high transparency in the IRT waveband, a paint system can be readily prepared with an IRT reflectivity of between 0.8 and 0.85. The disadvantage of Type 2 paints formed purely from appropriately sized metal flakes such as 30 μm diameter aluminium flakes is aesthetic, since they appear silver-metallic in colour when in a high IRT reflectivity formulation (or grey-metallic or ‘gold’ in the case of metal flakes based on coloured metals such as tungsten and brass respectively). By using small metal flakes (<5 μm diameter) or roughend flakes, grey paints can be made without a metallic appearance, but there is a significant reduction in IRT reflectivity due to scattering losses.
Type 3 IRT reflective paints, achieve a coloured effect, through the combination of metal reflector flakes and conventional granular visual colour pigments in a binder. DE10010538 (Hugo Gerd, “Coating composition having spectral selective properties, useful for the coating of buidlings, comprises four different particles having a range of wavelength dependant absorption properties”) describes a coating of this form. When the paint has dried, a thin layer of the binder polymer, loaded with the visual pigment, forms over the metal flakes to provide the visual colour; the ‘colour layer’.
The disadvantage of this approach is that when particulate colour pigments are added to flake-containing binders, the orientation of the flakes will be disturbed so that they no longer align with each other or with the surface of the paint. The mis-alignment reduces the IRT reflectivity achievable with the paint system due to scattering related effects. The thickness of the colour layer has to be controlled to retain IRT transparency to allow the IRT radiation to reach the reflector particles and be reflected back out. To achieve high IRT reflectivity (>0.7), the colour layer thickness should be less than 5 μm. To achieve moderate IRT reflectivity (>0.5), the colour layer thickness should be less than 10 μm. The durability of Type 3 paints using a thin colour layer is limited since the removal of the thin colour layer through scuffing and abrasive cleaning etc. can reveal the presence of the metal reflector layer causing the paint to appear ‘silvery’.